where each internal reaction \(r_j\)  in the network is split into its forward and backward component such that \(r_j= r_j^+- r_j^-\)  and \(r_j^+, r_j^-\ge 0\)  where \(a_j^+\)  and \(a_j^-\)  are normalized cost coefficients associated with each component of the reaction \(j\)  respectively.
From the mathematical point of view, it is important to note that these constraints define a convex space of feasible flux configurations [1]}.
